Counter pressure casting and counter pressure casting device

ABSTRACT

A counter pressure casting and a counter pressure casting device in which the pressures in a furnace side and a casting mold side are set to be lower than the maximum pressure of the process at a stage of the molten metal charging and increased to the maximum after the molten metal charging step. This technique provides a casting having less casting defects and improves the productivity.

FIELD OF THE INVENTION

This invention relates to a method for casting metal such as aluminumalloy, magnesium alloy, titanium alloy or the like, and particularly toa counter pressure casting and its device, wherein a moltenmetal-containing furnace and a casting mold are placed in airtightpressure containers, a gas having a pressure higher than the atmosphericpressure is charged into the containers, the pressure on the furnaceside is relatively increased more than the casting mold side, therebycharging the molten metal, into the casting mold.

BACKGROUND OF THE INVENTION

Casting defects such as pinholes, shrinkage cavity (porosity) or thelike due to solidification and shrinkage of the molten metal in theprocess of casting metals such as aluminum alloy, magnesium alloy,titanium alloy or the like air bubbles or hydrogen gas bubbles indendrite trees are generated in the solidification process and grow withthe solidification progress of the molten metal.

The hydrogen gas bubbles which form a nucleus of casting defects aregenerated when ambient pressure in a pressure container acting on themolten metal is lower than the hydrogen gas partial pressure in themolten metal, and the hydrogen gas partial pressure is sharply increasedas the liquid phase ratio lowers.

To prevent the hydrogen gas bubbles from forming, it is effective toapply ambient pressure higher than the hydrogen gas partial pressureonto the molten metal at the stage prior to the solidification of themolten metal. A casting method disposing a casting mold and a furnace inairtight pressure containers and applying higher pressure than theatmospheric pressure to the pressure containers was invented in Bulgariain the 1960s, which is widely known as Counter Pressure Casting.

In this counter pressure casting, as shown in FIG. 18 describingpressure control pattern, the holding furnace and the casting mold inthe pressure containers are applied the same pressure from theatmospheric pressure to the set pressure P1, then, the pressure in thecasting mold side is lowered while keeping that in the holding furnaceside at the set level by which the molten metal starts to be chargedinto the casting mold. Then, after the charging of the molten metal iscompleted at T2, the pressures in both sides are maintained at certainlevels from T2 to T3. After T3, the casting mold side pressure isincreased to the holding furnace side pressure to dissolve thedifferential pressure. thereby the molten metal is returned to theholding furnace at T4. Further, after T4, the process discharging thegas from the pressure containers to the atmosphere starts to completethe casting of one cycle at T5.

As to the above Counter Pressure Casting, Japanese Patent Laid-open No.186259/1989 and Japanese Patent Laid-open No. 278949/1989 disclosedcasting methods characterized by providing the differential pressurebetween the casting mold side and the holding furnace side at from 0.5to 30% of the maximum pressure; a method applying and holding thepressure of from 3 to 7 kgf/cm² to the pressure containers, thenadjusting the differential pressure at from 3 to 30% of the holdingpressure; and a method increasing and holding the pressure of thecontainers at from 7 to 30 kgf/cm², then adjusting the differentialpressure at from 0.5 to 10% of the holding pressure. Further, JapanesePatent Laid-open No. 187247/1990 disclosed a casting methodcharacterized by a following pressure controlling: applying a givenpressure to both containers, retention thereof, generation of thedifferential pressure between both containers and its retention, anddecrease of pressure to the atmospheric pressure.

But, the above conventional counter pressure casting has the followingproblems.

As shown in FIG. 18, the pressures of the furnace side and casting moldside have to be previously increased go P1 which is the maximum pressurein the process before T1 when the molten metal starts to be charged intothe casting mold. This makes the duration till T1 long resulting in itslow productivity in industrial application.

In order to decrease the period till T1 to improve the productivity, anair current has to be blown into the both containers at a high speed, bywhich the molten metal within the furnace side container is stirredcausing the generation of oxides of the molten metal. Such oxides aremixed with the casting as non-metal inclusions providing thedeterioration of the casting. Such non-metal inclusions cause internalor external defects and poor strength in the casting.

Further, the conventional counter pressure casting mentioned above is amethod to charge the molten metal into the casting mold by either of thecasting mold side pressure reduction method or the furnace side pressureincrease method, and the differential pressure is to increase whileforming a simple primary curve, Since the differential pressureincreasing speed is kept statically even after the completion ofcharging of the molten metal, unequal solidification proceeds, andfeeding head effect from the holding furnace side cannot be expected. Asa result, casting defects are left behind to cause internal or externaldefects and poor strength in the product.

Such defects become particularly obvious when a thin-wall part with acomplicated shape, thick-wall part or a material that is difficult tocast is cast, and can not be removed completely.

In addition, in the conventional counter pressure casting shown in FIG.18, the compressed pressure in both containers is entirely dischargedinto the atmosphere through an exhaust pipe after solidifying so thatthe molten metal is returned to the holding furnace. Therefore. Themolten metal moves up and down between x in FIG. 19 showing aconventional counter pressure casting device for every casting cycle,wherein x represents the distance between the molten metal surface inthe furnace and the molten metal highest position in the casting mold.As the above casting work progresses, the surface of the molten metal inthe furnace gradually lowers resulting in appearance of y in FIG. 19representing the difference between the initial and final height of themolten metal in the furnace. This causes the molten metal temperature,and the necessary charging pressure and time to be changed, andaccordingly, affects the quality of the casting as dispersing.

Furthermore, as a result of the molten metal moving up and down withinmolten metal feeding pipe, turbulent flow is induced in the molten metalin the furnace causing gas entrainment and other defects.

SUMMARY OF THE INVENTION

An object of this invention is to provide a counter pressure casting andcounter pressure casting device which can improve productivity byshortening the casting cycle time when applied to industrial production.

Another object of this invention is to provide a counter pressurecasting and counter pressure casting device capable of stabilizing thecasting conditions so as to obtain casting with little casting defectsor little non-metallic inclusion when a thick-wall casting or thin-wallcasting with a complicated shape is produced or when a material that isdifficult to cast is used.

It has now been found that the objects of the present invention can beattained when the pressures in both containers are held lower than themaximum pressure at the stage before the start of charging of the moltenmetal into the casting mold, and when they are increased to the maximumin the process of applying and holding the pressure to make thedifferential pressure after charging of the molten metal.

Further, the objects of the present invention can be achieved when apressure slightly higher than the atmospheric pressure is always appliedto the furnace side container to position the molten metal surfaceslightly lower than the sprue of the casting mold in the molten metalfeeding pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing one example of the pressure control patternwith respect to the holding furnace side container and the casting moldside container according the present invention.

FIG. 2 is a graph showing a differential pressure pattern generated dueto the pressure control pattern shown in FIG. 1.

FIG. 3 is a graph showing another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 4 is a graph showing yet another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 5 is a graph showing another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 6 is a graph showing still another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 7 is a graph showing also another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 8 is a graph showing another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 9 is a graph showing another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 10 is a graph showing another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 11 is a graph showing still another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 12 is a graph showing another example of the pressure controlpattern with respect to the holding furnace side container and thecasting mold side container according to the present invention.

FIG. 13 is an explanatory view showing a counter pressure casting deviceof this invention.

FIG. 14 is a perspective view showing a casting produced by the counterpressure casting of this invention.

FIG. 15 is a graph showing the pressure control pattern with respect tothe holding furnace side container and the casting mold side containeraccording to the present invention in view of the relation with thetemperature changes of the molten metal in the casting mold.

FIG. 16 is a photograph showing a cross sectional microstructure of thecasting prepared by the present invention. (magnification 100 times)

FIG. 17 is a photograph showing a cross sectional microstructure of thecasting of the comparative example. (magnification 100 times)

FIG. 18 is a graph showing the pressure control, pattern with respect tothe holding furnace side container and the casting mold side containeraccording to the conventional counter pressure casting.

FIG. 19 is an explanatory view showing a conventional counter pressurecasting device.

DETAILED DESCRIPTION OF THE INVENTION

The counter pressure casting according to this invention communicatesthe casting mold side pressure container having a casting mold disposedinside and the holding furnace side pressure container having a furnacecontaining molten metal disposed inside by means of a molten metalfeeding pipe, and is characterized by:

(1) "molten metal charging step" to charge the molten metal into thecasting mold under a pressure lower than the maximum pressure in thecontainers,

(2) "container interior pressure increasing step" to increase thepressures in the holding furnace side container and the casting moldside container,

(3) "differential pressure holding step" to hold the differentialpressure between both containers of the holding furnace side and thecasting mold side at a certain pressure,

(4) "differential pressure dissolving step" to dissolve differentialpressure between the both pressure containers, and

(5) "pressure reducing step" to reduce the pressures in both containersto certain ambient pressures.

In the above counter pressure casting, it is preferable that a pressureslightly higher than the atmospheric pressure is always applied to theholding furnace side container, thereby positioning the molten metalsurface slightly lower than the sprue of the casting mold in the moltenmetal feeding pipe.

In this case, the pressure always applied to the holding furnace sidecontainer is desirably changed as required based on (1) properties ofthe molten metal, (2) characteristics of the device and others, and (3)the distance between the molten metal surface in the molten metalfeeding pipe 5 and the remaining molten metal surface in the holdingfurnace. A pressure mentioned above is determined, and successivelyincreased with the progress of the casting work so as to maintain thesurface of the molten metal in the molten metal feeding pipe 5 to be ata certain level.

The maximum pressure in the containers is set by various conditions suchas the composition of molten metal, the usage of the product, the shapeof the product and the like, and defined as the maximum value of theabsolute pressure in the holding furnace side container and/or castingmold side container based on the atmospheric pressure in the castingprocess of one cycle.

The container interior pressure at the start of charging the moltenmetal into the casting mold is preferably set to 0 to 50% of the abovemaximum pressure, and more preferably to 10 to 30% of the maximumpressure. When it exceeds 50% of the maximum pressure, it is necessaryto increase the rate of air current blown into the container to reducethe casting cycle time, and as a result, some characteristics of theproduct are deteriorated by stirring and oxidation of the molten metalcaused by the air current. If it is less than 10% of the maximumpressure, casting defects in the product can not be removed completely.On the other hand, when it exceeds 30% of the maximum pressure,inclusions may be mixed into the casting, possibly lowering the strengthof the product.

According to the present invention, generation and increase of thedifferential pressure in the molten metal charging step include thefollowing embodiments and they are suitably combined for practicedepending on the use, the material and the shape of the desired casting.

(1) Holding furnace side pressure increase with casting mold sideconstant pressure

(2) Holding furnace side pressure increase with casting mold sidepressure increase

(3) Holding furnace side pressure increase with casting mold sidepressure decrease

(4) Holding furnace side constant pressure with casting mold sidepressure decrease

(5) Holding furnace side pressure decrease with casting mold sidepressure decrease

In the above embodiments, when the molten metal charging step is acomposite step including a step increase a pressure in the holdingfurnace side container and a step to increase a pressure in the castingmold side container, differential pressures between the both containersis generated relatively by the difference of the pressure increasebetween both containers, thereby the texture of the product can be madefine as the absolute pressure can be set higher and casting defectoccurrence can be prevented as the differential pressure increasingspeed is varied in high velocity.

When the molten metal charging step is determined to be a step to lowerthe pressure in the casting mold side container, the molten metal is fedinto the casting mold by the suction force and a run into the castingmold can be improved.

Further, in any of the above embodiments, the molten metal charging stepcan be designed to consist of the first step molten metal chargingprocess and the second step molten metal charging process which has agreater differential pressure increase than the first step process.Thus, a desirable product having no defects can be produced becauseapplying a high differential pressure which is required according to theprogress of solidification of the molten metal and the feeding headeffect caused by the impact on the differential pressure change pointfrom the first step process to the second step process. In addition,generation of solidification nucleus is accelerated, thereby much finertexture can be obtained.

The differential pressure change point from the above first step processto the second step process can be set at the completion of charging themolten metal into the casting mold. Thus, the differential pressuresrequired respectively in the process of the molten metal feeding processinto the casting mold and the solidification process after thecompletion of molten metal charging can be efficiently and effectivelyapplied. In order to know when the molten metal charging is complete, aplurality of thermo couples disposed on the casting mold cavity surfacecan be used by detecting a temperature change.

In the above case, the embodiment of generation and increase of thedifferential pressure on the first step molten metal charging processand the second step molten metal charging process has the followingvariations and they are suitably combined for practice depending on theuse, the material and the shape of the desired casting.

(1) Holding furnace side pressure increase with casting mold sideconstant pressure

(2) Holding furnace side pressure increase with casting mold sidepressure increase

(3) Holding furnace side pressure increase with casting mold sidepressure decrease

(4) Holding furnace side constant pressure with casting mold sidepressure decrease

(5) Holding furnace side pressure decrease with casting mold sidepressure decrease

The differential pressure between the holding furnace side container andthe casting mold side container in the above steps are designed to forma non-linear curve in a pressure-time curve which shows non-proportionalrelation with the passage of time, thereby desirable pressure controlcan be achieved to prevent the molten metal in the furnace ofunnecessary air blowing caused by the increase of pressure, and thecasting cycle time can be shortened. As a result, the product withpreferable properties depending on the kind, the use and the shape canbe obtained.

Aforementioned "differential pressure holding step" can be varied as aprocess to increase the pressures in both containers of the holdingfurnace side and casting mold side having constant differentialpressure. Further, it can be made to consist of the first stepdifferential pressure holding process to increase the pressures in bothcontainers of the holding furnace side and casting mold side havingconstant differential pressure, and the second step differentialpressure holding process to hold constant pressures in both containersrespectively. Thus, by continuing the pressure increase of bothcontainers even in the differential pressure holding step, thecharacteristics of the product are improved, especially the toughness isincreased as the crystalline particles of the product are made fine.

According to the counter pressure casting of this invention, quite goodcharacteristics can be obtained for the thin wall portion of a casting.But, the thick wall portion or a thin-wall portion with a complicatedshape may suffer from the concentration of casting defects.

As a result of further study, it has now been found that concentrationof casting defects mentioned above is caused due to an application of ahigh pressure to the holding furnace side container and the casting moldside container in an early stage of casting. Based on such study, ameasure to prevent the concentration of casting defects on a thick-wallportion or a thin-wall portion with complicated shape was taken asfollows.

According to the counter pressure casting of this invention, it ispreferable to hold the pressure in the casting mold side container to below for a prescribed time after the completion of charging of the moltenmetal.

The above low pressure is defined as a pressure 0 to 3 kg/cm² higherthan the atmospheric pressure, and its low pressure holding time can beset as a time when the solidification of a desired part of the castingcompletes. Thereby, the concentration of casting defects on a certainpart can be prevented, and the strength of the product can be improvedefficiently.

According to the counter pressure casting of this invention, it is alsodesirable to hold the pressure in the holding furnace side container andthe casting mold side container to a certain pressure for a certain timeafter the completion of charging of the molten metal.

The certain pressure holding time mentioned above can be set as a timewhen solidification at a desired part of the casting is complete.

The certain part mentioned above can be set as a portion required tohave strength particularly when the subject casting is actually used, ora portion which is recognized by experience to have the generation offaults due to the concentration of casting defects.

Accordingly, by the counter pressure casting of this invention, acasting without localized concentration of casting defects can beobtained even when a particularly thick wall casting or the thin-wallcasting with a complicated shape is produced.

The counter pressure casting device of this invention comprises acasting mold side pressure container having a casting mold disposedinside, a holding furnace side pressure container having a furnacecontaining molten metal inside, a molten metal feeding pipe forconnecting the furnace interior and the casting mold interior, apressure means to respectively increase the pressure in the holdingfurnace side container and the casting mold side container to exceed theatmospheric pressure, and pressure control means having a function tocontrol the pressures in the casting mold side container and the holdingfurnace side container to be lower than the maximum pressure in thecontainers at the time of starting the molten metal charging process.

The above counter pressure casting device further includes a means todetect charging of the molten metal into the casting mold. The abovepressure control means has a function to vary a differential pressureincreasing speed between the casting mold side container and the holdingfurnace side container in combination with the charging detection means.

In addition, the above pressure control means has a function to alwaysapply slightly higher pressure than the atmospheric pressure to theholding furnace side container.

The above pressure control means is preferably set so that the pressuresin the casting mold side container and the holding furnace sidecontainer are controlled to 0 to 50% of the maximum pressure when themolten metal charging process is started.

Additionally, the above pressure control means can be set as the castingmold side container has a pressure substantially the same as theatmospheric pressure while the pressure in the holding furnace sidecontainer is increased when charging of the molten metal into thecasting mold starts, then both containers are kept at certain pressuresfor a certain period. This prevents casting defects from concentratinginto the thick portion of the product.

Further, concentration of casting defects into the thick portion of theproduct can be also prevented by setting the above pressure controlmeans to hold the casting mold side container to a low pressure for acertain period before simultaneously increasing the pressures of bothcontainers after charging the molten metal into the casting mold.

The aforementioned counter pressure casting and counter pressure castingdevice of this invention will be specifically described with referenceto the drawings.

FIG. 1 shows one example of the pressure control pattern in the castingmold side container and the holding furnace side container by thisinvention, and FIG. 2 shows a differential pressure pattern between bothcontainers generated by the pressure control pattern of FIG 1. In FIG.1, FIG. 3 through FIG. 12 and FIG. 18, the solid line shows the pressurepattern in the furnace side container and the dotted line shows that inthe casting mold side container. Both containers have their pressuresincreased to P1 from the start of casting to T1, and the casting moldside container has its pressure decreased to P2 while keeping thepressure of the holding furnace side container at P1, thereby feedingthe molten metal into the casting mold.

Providing that a time when charging of the molten metal into the castingmold is confirmed is determined to be T2, from T2 to T3, the pressure inthe casting mold side container is kept at P2 and at the same time, theholding furnace side is gradually increased up to P3 to increase thedifferential pressure and to enhance the feeding head effect, therebythe molten metal is supplied to surroundings of crystals appearing inthe process of solidification resulting in preventing casting defects.

Then, after increasing the holding furnace side pressure to a certainpressure P3, the pressures are kept in both containers from T3 to T4respectively, thereby holding the constant differential pressure betweenboth containers. Further, after T4, the holding furnace side pressure P3is lowered to be equal with the casting mold side pressure P2, and thedifferential pressure is dissolved at T5 to return the molten metal tothe holding furnace, and after T5, the gas in the containers isdischarged to the atmosphere to return to the atmospheric pressure P0and finally, the casting of one cycle is completed.

With the above pressure control pattern, as shown in FIG. 1, thepressures in both containers before starting the molten metal feedingare increased to P1, which is lower than the maximum pressure P3 of theholding furnace side container. Thus, the casting cycle time isshortened and oxidation of the molten metal in the furnace is preventedin the pressure increasing process, thereby obtaining a good casting.

With the above pressure control pattern, the differential pressureincreasing speed (ΔP/ΔT) is set larger from T2 to T3 than from T1 to T2as shown in FIG. 2. In the above process, the pressures in bothcontainers and the differential pressure are always monitored from thestart to the end of the one cycle casting. The measured values arealways fed back to the pressure control means and if the measured valuesexceed the set values, an exhaust valve is opened to discharge from thecontainers, so that the pressures in the containers are always kept atthe set values.

FIG. 3 shows another example of the pressure control pattern of thecasting mold side container and the holding furnace side containeraccording to this invention. The pressures of both containers areincreased to P1 from the start of casting to T1, then the pressure ofthe casting mold side is lowered to P2 while keeping the pressure of theholding furnace side at P1 from T1 to T2, thereby feeding the moltenmetal into the casting mold. That is, the molten metal in the holdingfurnace rises in the feeding pipe to be charged in the casting mold. Thecharged molten metal is cooled by releasing heat to the casting mold andsolidification progresses from the position separated from the spruetoward the sprue with time.

Providing that a time when the charging of the molten metal into thecasting mold is confirmed is determined to be T2, the pressure in thecasting mold side container is more quickly decreased to P3 from T2 toT3, then after decreasing the casting mold side pressure to P3, thepressures in both containers are simultaneously increased at the samespeed in the range of T3 to T4, thereby holding the differentialpressure between both containers at a certain level. Then, after T4,both containers are held at certain pressures respectively to keep thedifferential pressure. Further, after T5, the holding furnace sidepressure is decreased to the same level with the casting mold sidepressure, and at T6, the differential pressure dissolved and the moltenmetal is returned to the holding furnace, then after T7, the gas in thecontainers is discharged into the atmosphere to return to theatmospheric pressure and the casting of one cycle is completed.

In the above pressure control pattern, the pressures in both containersat the start of feeding of the molten metal are P1, and this correspondsto about 30% of the maximum pressure Pf-max (PS) in the holding furnaceside container. Thus, the casting cycle time is shortened and at thesame time, oxidation of the molten metal in the furnace on the pressureincreasing process is prevented, thereby obtaining a good casting. Thispressure control pattern is applied when a strong part such as analuminum wheel is cast by using the molten metal of Al--Si--Mgcomposition.

When pressures P1 in both containers are set to about 20% of the maximumpressure Pf-max (P5) in the holding furnace side container at the startof feeding in the above pressure control pattern, this pattern isapplicable when a corrosion resistant part is cast by using the moltenmetal of Al--Mg composition.

FIG. 4 shows another example of the pressure control pattern of thecasting mold side container and the holding furnace side containeraccording to this invention. Both containers have the pressuresincreased to P1 in a range from the start to T1, then the pressure ofthe casting mold side is lowered to P2 while keeping the pressure of theholding furnace side at P1. Thus, the molten metal in the holdingfurnace rises in the feeding pipe and is charged in the casting mold,and the charged molten metal is cooled by releasing heat to the castingmold and starts solidification.

Then, providing that a time when the charging of the molten metal intothe casting mold is confirmed is determined to T2, the pressures in bothcontainers are increased in a range from T2 to T3 respectively, and thedifferential pressure is increased by the difference of pressureincreasing degree. Then, after increasing both container pressures to P3and P4 respectively, both containers are kept unchanged from T3 to T4 tohold the differential pressure at a certain level. Then, after T4, theholding furnace side pressure is lowered to be identical with thecasting mold side pressure, and at T5, the differential pressure isdissolved and the molten metal is returned to the holding furnace, andafter T6, the gas in the pressure containers is discharged into theatmosphere to return to the atmospheric pressure and the casting of onecycle is completed.

In above pressure control pattern, the pressures in both containers atthe start of feeding are P1, and this corresponds to about 30% of themaximum pressure Pf-max (P4) of the holding furnace side container. Thispressure control pattern is suitable when a complicated-shaped part suchas an engine block is cast using the molten metal of Al--Si--Cucomposition.

FIG. 5 shows still another example of the pressure control pattern ofthe casting mold side container and the holding furnace side containeraccording to this invention. Both containers have the pressuresincreased to P1 in a range from the start to T1, then the pressureincreasing speed of both containers is lowered and at the same time, thedifferential pressure is generated relatively between both containers bythe difference of the pressure increase degree in both containers. Thus,the molten metal in the holding furnace rises the feeding pipe and ischarged into the casting mold.

Then, the casting mold side pressure container and the holding furnaceside pressure container are respectively increased to P2 and P3.Providing that a time when charging of the molten metal into the castingmold is confirmed is determined to be T2, from T2 to T3, both containershave pressures quickly increased to P4 and P5 respectively and accordingto the difference of pressure increasing degree, the differentialpressure is more increased, then both containers are kept constant fromT3 to T4 to hold the differential pressure at a certain level. Then,after T4, the holding furnace side pressure is lowered to be identicalwith the casting mold side pressure, and at T5, the differentialpressure is resolved and the molten metal is returned to the holdingfurnace. After T6, the gas in the pressure containers is discharged tothe atmosphere to return to the atmospheric pressure and the casting ofone cycle is completed.

In the above pressure control pattern, the pressures in both containersat the start of molten metal feeding are P1, and this corresponds toabout 30% of the maximum pressure Pf-max (P5) of the holding furnaceside container. This pressure control pattern is applied when a thickwall part is cast by using the molten metal of Al--Cu composition.

FIG. 6 shows still another example of the pressure control pattern ofthe casting mold side container and the holding furnace side containeraccording to this invention. Both containers have the pressuresincreased to P1 in a range from the start to T1, then the pressure ofthe casting mold side is lowered to P2 while keeping the pressure of theholding furnace side at P1, thereby feeding the molten metal into thecasting mold. Then, after the casting mold side is decreased to P2, thepressure in the casting mold side container is more quickly lowered toP3 from T2 to T3. Then, both containers have the pressures increasedsimultaneously at the same speed from T3 to T4, thereby the differentialpressure between both containers is kept at a certain level. Then, afterT4, the holding furnace side pressure is lowered to be identical withthe casting mold side pressure, and at T5, the differential pressure isdissolved and the molten metal is returned to the holding furnace.Further, both containers have the pressures lowered to P4, and after T6,both container pressures are held at a certain level. Further, after T7,the gas in both containers is discharged to the atmosphere to return tothe atmospheric pressure and the casting of one cycle is completed.

In the above pressure control pattern, the pressures of both containersat the start of the molten metal feeding are P1, and this correspond toabout 20% of the maximum pressure Pf-max (P6) of the holding furnaceside container. This pressure control pattern is applicable when a largestrong part is cast by using the molten metal of Al--Cu--Mg composition.

In the above pressure control pattern, the maximum pressure of theholding furnace side container is Pf-max (P6), and the maximum pressureof the casting mold side container is Pm-max (P5). These are set highercompared with the maximum pressure of other pressure control patternsmentioned before. Crystalline particles of the casting can be made fineand its toughness is to be enhanced by applying such a high pressure inthe solidification process of the molten metal.

FIG. 7 shows another example of the pressure control pattern of thecasting mold side container and the holding furnace side containeraccording to this invention. The holding furnace side container isalways applied with a certain pressure higher than the atmosphericpressure, and the free surface of the molten metal is designed to bepositioned slightly lower than the sprue of tile casting mold in themolten metal feeding pipe. The holding furnace side container has itspressure increased to P1 at the same pressure increasing speed with thatof the casting mold side container in a range from the start to T1, thenthe pressure of the casting mold side is lowered to P2 while keeping thepressure of the holding furnace side at P1. Thus, the molten metal isfed into the casting mold, and charged molten metal is cooled byreleasing heat to the casting mold to progress solidification.

Then, providing that a time when the charging of the molten metal intothe casting mold is confirmed is determined to be T2, the pressure inthe casting mold side container is more quickly lowered to P3 in a rangefrom T2 to T3. The pressures of both containers are increasedsimultaneously at the same speed from T3 to T4, thereby maintaining thedifferential pressure of both containers to a certain level. Then afterT4, both containers are retained at P4 and P5 respectively to keep theconstant differential pressure. Further, after T5, the holding furnaceside pressure is lowered to a pressure which is about 0.15 kgf/cm²higher than the casting mold side pressure to return the molten metal sothat the molten metal free surface is positioned near the sprue of thecasting mold in the molten metal feeding pipe. After T6, the pressure inthe holding furnace side container is decreased to a certain levelhigher than the atmospheric pressure and the pressure in the castingmold side container is decreased to the atmospheric pressure.

In the above control pattern, the pressure of the holding furnace sidecontainer is increased to P1 before the start of molten metal feeding,and this corresponds to a range of 15 to 40% of the maximum pressurePf-max (P5) of the holding furnace side container. This pressure controlpattern is applied to the casting of an aluminum wheel by using themolten metal of Al--Si--Mg composition.

In the above pressure control pattern, when pressure P1 of the holdingfurnace side container is set in a range of 5 to 25% of the maximumpressure Pf-max (P5) of the holding furnace side container, such apressure control pattern is applicable to the casting of a corrosionresistant part by using the molten metal of Al--Mg composition.

FIG. 8 shows another example of the pressure control pattern of thecasting mold side container and the holding furnace side containeraccording to this invention. The holding furnace side container, whichis always applied with a certain pressure more than the atmosphericpressure so that the molten metal free surface is positioned near thesprue of the casting mold in the molten metal feeding pipe, has thepressure increased to P1 at the same increasing speed with the castingmold side from the start to T1, then the pressure of the casting moldside is lowered to P2 keeping the pressure of the holding furnace sideat P1. Thereby, the molten metal is fed into the casting mold and thecharged molten metal is cooled by releasing heat to the casting mold tostart solidification.

Then, providing that a time when the charging of the molten metal intothe casting mold is confirmed is determined to be T2, the pressures inboth containers are increased to P3 and P4 respectively in a range fromT2 to T3, and by the difference of pressure increase degree, thedifferential pressure is quickly increased. Then, the pressures of bothcontainers are held at P3 and P4 respectively from T3 to T4 to hold thedifferential pressure at a certain level. Then, after T4, the holdingfurnace side pressure is lowered to a pressure which is about 0.15kgf/cm² higher than the casting mold side pressure to return the moltenmetal so that the free surface of the molten metal is positioned nearthe sprue of the casting mold in the molten metal feeding pipe. AfterT6, the pressure in the holding furnace side container is decreased to acertain level higher than the atmospheric pressure and the pressure inthe casting mold side container is decreased to the atmosphericpressure.

With the above pressure control pattern, the pressure in the holdingfurnace side container is increased to P1 before starting the feeding ofmolten metal and this corresponds to a range of 5 to 25% of the maximumpressure Pf-max (P4) of the holding furnace side container. Thispressure control pattern is particularly suitable for casting an engineblock by using the molten metal of Al--Si--Cu composition.

FIG. 9 shows still another example of the pressure control pattern ofthe casting mold side container and the holding furnace side containeraccording to this invention. The holding furnace side container, whichis always applied a certain pressure over the atmospheric pressure sothat the molten metal free surface is positioned near the sprue of thecasting mold in the molten metal feeding pipe, has the pressureincreased to P1 at the same increasing speed with the casting mold sidefrom the start to T1, then the pressure increasing speed in bothcontainers is lowered and the differential pressure is relativelygenerated between both containers depending on the difference of thepressure increase degree. Thus, the molten metal is fed into the castingmold.

Then, providing that a time when the charging of the molten metal intothe casting mold is determined to be T2, both containers are quicklyincreased their pressures to P3 and P4 respectively and the differentialpressure is further increased depending on the difference of thepressure increasing degree from T2 to T3, then after T3, both containersare kept statically from T3 to T4 to keep the differential pressure at acertain level. Then, after T4, the holding furnace side is lowered to apressure about 0.15 kgf/cm² higher than the casting mold side pressureso that the molten metal is returned and its free surface is positionednear the sprue of the casting mold in the molten metal feeding pipe.After T5, the pressure in the holding furnace side container isdecreased to a certain level higher than the atmospheric pressure andthe pressure in the casting mold side container is decreased to theatmospheric pressure.

In the above pressure control pattern, the pressure in the holdingfurnace side container before the start of the molten metal feeding isincreased to P1, and this corresponds to a range of 5 to 25% of themaximum pressure Pf-max (P4) in the holding furnace side container. Thispressure control pattern is particularly suitable for casting a thickwall part by using the molten metal of Al--Cu composition.

FIG. 10 also shows another example of the pressure control pattern ofthe casting mold side container and the holding furnace side containeraccording to this invention. The holding furnace side container, whichis always applied a certain pressure over the atmospheric pressure sothat the molten metal free surface is positioned near the sprue of thecasting mold in the molten metal feeding pipe, has its pressureincreased to P1 at the same pressure increasing speed with that in thecasting mold side container in a range from the start to T1, then thepressure of the casting mold side is lowered to P2 while keeping thepressure of the holding furnace side at P1. Thus, the molten metal isfed into the casting mold.

Then, providing that a time when the charging of the molten metal intothe casting mold is determined to be T2, the pressure in the castingmold side container is more quickly lowered to P3 in a range from T2 toT3. The pressures of both containers are increased simultaneously at thesame speed from T3 to T4, thereby maintaining the differential pressurebetween both containers to a certain level. Then, after T4, bothcontainers have lowered their pressures keeping the differentialpressure at the same level. The pressure reduction of the casting moldside is stopped at T5, while the pressure of the holding furnace side iscontinuously lowered so that the differential pressure of about 0.15kgf/cm² is formed, thereby the molten metal is returned and its freesurface is positioned near the sprue of the casting mold in the moltenmetal feeding pipe. After T6, both container pressures are retained at acertain level. Further, after T7, the pressure in the holding furnaceside container is decreased to a certain level higher than theatmospheric pressure and the pressure in the casting mold side containeris decreased to the atmospheric pressure.

In the above pressure control pattern, the pressure of the holdingfurnace side container is increased to P1 before starting the moltenmetal feeding, and this corresponds to a range of 5 to 25% of themaximum pressure Pf-max (P6) of the holding furnace side. This pressurecontrol pattern is particularly suitable for casting a strong part byusing the molten metal of Al--Cu--Mg composition.

In the above pressure control pattern, the maximum pressure of theholding furnace side container is Pf-max (P6), and the maximum pressureof the casting mold side container is Pm-max (PS). These are set highercompared to the maximum pressures of other pressure control patternsmentioned before. Crystalline particles of the casting can be made fineand its toughness is to be enhanced by applying such a high pressure inthe solidification process of the molten metal.

FIG. 11 shows another example of the pressure control pattern of thecasting mold side container and the holding furnace side containeraccording to this invention. The holding furnace side container, whichis always applied a certain pressure over the atmospheric pressure sothat the molten metal free surface is positioned near the sprue of thecasting mold in the molten metal feeding pipe, has its pressureincreased to P1 for from T1 to T2. Thus, the molten metal is fed intothe casting mold. Then, providing that a time when the charging of themolten metal into the casting mold is determined to be T2, the pressureof the holding furnace side is retained at P1 to apply the constantpressure to the charged molten metal for from T2 to T3.

Then, the pressures of the holding furnace side container and thecasting mold side container are simultaneously increased at the samespeed, thereby keeping a certain differential pressure between bothcontainers from T3 to T4. Then, both containers have their pressureskept statically so that the differential pressure is kept the same fromT4 to T5. After T5, the holding furnace side pressure is lowered to apressure about 0.15 kgf/cm² higher than the casting mold side pressure,thereby the molten metal is returned and the molten metal free surfaceis positioned near the sprue of the casting mold in the molten metalfeeding pipe. After T6, the pressure in the holding furnace sidecontainer is decreased to a certain level higher than the atmosphericpressure and the pressure in the casting mold side container isdecreased to the atmospheric pressure.

FIG. 12 shows another example of the pressure control pattern of thecasting mold side container and the holding furnace side containeraccording to this invention. The holding furnace side container isalways applied a certain pressure higher than the atmospheric pressureso that the free surface of the molten metal is positioned near thesprue of the casting mold in the molten metal feeding pipe. The holdingfurnace side container has its pressure increased to P1 at the samepressure increasing speed with that of the casting mold side containerin a range from the start to T1, then the pressure of the casting moldside is lowered to P2 while keeping the pressure of the holding furnaceside at P1 from T1 to T2, Thus, the molten metal is fed into the castingmold.

Then, the casting mold side pressure is more quickly lowered to P3 in arange from T2 to T3. Providing that a time when the charging of themolten metal into the casting mold is determined to T3, the pressure inthe casting mold side is kept at a low pressure of 0 to 3 kg/cm ² toapply such a low pressure to the charged molten metal until T4. Thepressures of both containers are respectively increased to P4 and P5 atthe same speed from T4 to T5, thereby maintaining the differentialpressure between both containers. Then, after T5, both containers areretained at P4 and P5 to keep the constant differential pressure.Further, after T6, the holding furnace side pressure is lowered to apressure which is about 0.15 kgf/cm² higher than the casting mold sidepressure to return the molten metal so that the molten metal freesurface is positioned near the sprue of the casting mold in the moltenmetal feeding pipe. After T7, the pressure in the holding furnace sidecontainer is decreased to a certain level higher than the atmosphericpressure and the pressure in the casting mold side container isdecreased to the atmospheric pressure.

According to the methods shown in FIG. 11 and FIG. 12, the pressureincrease to the maximum pressures in both containers is effected afterthe solidification of a portion where local concentration of castingdefects tend to be observed, for example a thin wall portion with acomplicated shape. This contributes to the prevention of such defectscaused by the pressure increase in an early stage before thesolidification of such a portion, thereby a good casting free fromdefects is obtained.

the counter pressure casting by the pressure control pattern shown inthe above FIG. 7 to FIG. 12, the casting cycle time Tp is determined asthe sum of the casting time Ta and the casting removal time Tb. Thecasting time Ta can be made shorter than a conventional casting time Tcshown in FIG. 18 because the holding furnace side container interior isalways held above the atmospheric pressure resulting in the totalcasting cycle time shortening.

As described above, according to the counter pressure casting andcounter pressure casting device of this invention, the followingadvantages are obtained in the casting process.

By communicating the casting mold side container and the holding furnaceside container and increasing the pressures of both containers to acertain level, the nucleus generation of the hydrogen gas in the moltenmetal is controlled, then with generating and increasing thedifferential pressure between both containers by reducing the castingmold side pressure after the communication valve of each pressurecontainer is closed to separate each other, the molten metal is fed tothe casting mold by the suction force due to the differential pressure.Thus, the run of the molten metal into the casting mold is remarkablyimproved. In addition, high feeding head effect can be obtained byrelatively increasing the internal pressure of the holding furnace sidecontainer, which contributes to the prevention of casting defectsoccurrence at the time of solidification, and a good casting can beobtained.

The absolute pressures which are needed in the casting within bothpressure containers are applied in the process after charging the moltenmetal into the casting mold, and the pressures in the containers at thestart of molten metal charging into the casting mold is set to a lowpressure compared with the maximum pressure. Thereby, the casting cycletime can be shortened because less pressurizing time in the containerbefore the molten metal charging is necessary compared with theconventional counter pressure casting and also adverse effects such asstirring and oxidation of the molten metal in the furnace can beprevented as the velocity of the air current blown into the holdingfurnace side container and the gas volume can be made small before thestart of molten metal charging.

The charged molten metal in the casting mold starts to solidify and theouter skin is formed near the casting mold. When the outer skin is onceformed, the action of the suction force to the molten metal is lowered,resulting in poor run of the molten metal to the casting mold. In orderto avoid such a problem, upon the completion of feeding the molten metalinto the casting mold, the casting mold side container is more quicklylowered to improve the differential pressure increasing speed betweenthe casting mold side container and the holding furnace side containerso that the action of the suction force to the molten metal ismaintained and the run of the molten metal to the casting mold is kept.

Similarly, when the inner pressure of the holding furnace side containeris relatively increased after charging the casting mold with the moltenmetal, a high feeding head effect can be obtained. Thus, the castingdefects can be prevented from occurring during solidification, resultingin a good casting.

Further, by maintaining the pressure in the furnace side containerslightly higher than the atmospheric pressure after the completion ofcasting of one cycle, the free surface of the molten metal stays nearthe sprue of the casting mold in the molten metal feeding pipe. As aresult, a time loss due to the feeding and returning of the molten metalbetween the holding furnace and the casting mold can be remedied, andthe casting time can be made shorter than that by the conventionalmethod, resulting in remarkable shortening of the casting time.

In addition, stirring of the molten metal in the holding furnace due tothe returning of the molten metal from the casting mold to the holdingfurnace can be prevented, and casting conditions can be maintained asthe molten metal free surface is positioned statically.

Furthermore, by holding the pressure in the holding furnace sidecontainer at a certain level for a certain time or applying a lowpressure in a certain range to the casting mold side container after themolten metal charging, a high feeding head effect can be obtained, andcasting defects particularly observed in a thick wall section can beprevented during solidification, resulting in a good casting.

In addition, the pressure increase to the maximum pressures in bothcontainers is effected after the solidification of a portion where localconcentration of casting defects tend to be observed, for example a thinwall portion with a complicated shape. This contributes to theprevention of such defects, thereby a good casting free from defects isobtained.

The present invention is now described below based on specific examplesand embodiments, but the present invention is not to be construed asbeing limited thereto.

EXAMPLE 1

FIG. 13 shows the counter pressure casting device of this invention,wherein a casting mold 4 is disposed in a casting mold side pressurecontainer 1 and a holding furnace 3 in a holding furnace side pressurecontainer 2. The molten metal in the holding furnace 3 is fed into thecasting mold 4 through a feeding pipe 5, which is communicating theholding furnace 3 and the casting mold 4, by the differential pressurebetween the pressure containers 1 and 2. A plurality of thermocouples 6is disposed in the casting mold 4 to measure the surface temperature ofa casting, and measurements by these thermocouples 6 are inputted in apressure control device 7. Numbers and position of these thermocouples 6are determined according to the kind, the shape and the size of asubject casting. Generally, the thermocouples 6 are disposed at constantintervals according to a distance from a sprue to the farthest end inthe vertical cross section of the casting mold containing the sprue.

Pressure means 8 and 9 are disposed on the sides of the casting mold 4and the holding furnace 3, respectively.

Control signals are outputted from the pressure control device 7 to theabove pressure means 8 and 9, the pressurized gas from a pressurized gassource 10 is supplied to the casting mold side container 1 and theholding furnace side container 2 through the pressure means 8 and 9, andeach pressure of the containers 1 and 2 is independently controlled.

In addition, discharge means 11 and 12 are opened or closedindependently or as interlocked according to the signals from thepressure control device 7 to discharge gas from each container.

In the above, the pressure control device 7 is previously provided witha program to execute the following (1) through (4) independently or incombination partly/in all simultaneously.

(1) The pressures in the casting mold side container and holding furnaceside container are determined to be 0 to 50% of the maximum pressure ofboth pressure containers when feeding of the molten metal into thecasting mold is started.

(2) After the completion of feeding the molten metal into the castingmold 4, the differential pressure between both containers 1 and 2 isfurther increased at a high speed.

(3) A pressure slightly higher than the atmospheric pressure is arrangedto be applicable usually to the furnace side container.

(4) Before starting the molten metal charging process, the pressure ofthe casting mold side container 1 is kept at the atmospheric pressurewhile the holding furnace side container 2 only is pressurized to chargethe molten metal into the casting mold.

In each example, the pressures and differential pressure of the pressurecontainers are always monitored from the start to the completion of thecasting, and when the differential pressure or pressures exceed theprescribed value, the above discharge means 11 and 12 are released todischarge, and also the measurements are fed back to the above pressurecontrol device 7 to maintain the set pressure value.

Using the device shown in FIG. 13, an aluminum alloy casting wasproduced by being pressurized up to 6 kgf/cm ² in maximum according tothe pressure control pattern shown in FIG. 1. The pressures in bothcontainers 1 and 2 were increased up to 5 kgf/cm² in the process ofpressurizing from the atmospheric pressure to P1 by T1. Then, thepressure in the casting mold side was gradually lowered while keepingthe pressure in the holding furnace side so that the molten metal wasfed into the casting mold from T1 to T2. Then, providing that a timewhen the charge of the molten metal into the casting mold was confirmedby the thermocouples 6 disposed at the top of the cavity was determinedto be T2, the pressure in the casting mold side container was keptconstant from T2 to T5. The holding furnace side pressure was graduallyincreased and held at the maximum from T2 to T4. After T4, the holdingfurnace side pressure was lowered to the same level with the castingmold side pressure to resolve the differential pressure from T4 to T5,thereby the molten metal was returned to the holding furnace. After T5,the pressures in both containers were reduced to the atmosphericpressure discharging the gas in the containers to the atmosphere, thuscompleting the casting of one cycle.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

COMPARATIVE EXAMPLE 1

A casting was obtained by following the procedure of EXAMPLE 1 except ause of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

EXAMPLE 2-1

Using the device shown in FIG. 13, an aluminum wheel was cast bypressurizing up to 6 kgf/cm using aluminum alloy molten metal ofAl--Si--Mg composition with P1 as 30% (1.8 kgf/cm²) of P5 according tothe pressure control pattern shown in FIG. 3.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

EXAMPLE 2-2

Using the device shown in FIG. 13, a corrosion-resistant part was Vastfrom aluminum alloy molten metal of Al--Mg composition with P1 as 20% ofP5 according to the pressure control pattern shown in FIG. 3.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

EXAMPLE 2-3

Using the device shown in FIG. 13, an automobile engine block was castfrom aluminum alloy molten metal of Al--Si--Cu composition with P1 as30% of P4 according to the pressure control pattern shown in FIG. 4. Theresults obtained by evaluating characteristics of the casting are shownin Table 1.

EXAMPLE 2-4

Using the device shown in FIG. 13, an automobile thick wall part wascast from aluminum alloy molten metal of Al--Cu composition with P1 as30% of P5 according to the pressure control pattern shown in FIG. 5.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

EXAMPLE 2-5

Using the device shown in FIG. 13, an automobile large-sized tough partwas cast from aluminum alloy molten metal of Al--Cu--Mg composition withP1 as 20% of P6 according the pressure control pattern shown in FIG. 6.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

COMPARATIVE EXAMPLE 2-1

A casting was produced by following the procedure of EXAMPLE 2-1 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

COMPARATIVE EXAMPLE 2-2

A casting was produced by following the procedure of EXAMPLE 2-2 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

COMPARATIVE EXAMPLE 2-3

A casting was produced by following the procedure of EXAMPLE 2-3 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

COMPARATIVE EXAMPLE 2-4

A casting was produced by following the procedure of EXAMPLE 2-4 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting.

The results obtained by evaluating characteristics the casting are shownin Table 1.

COMPARATIVE EXAMPLE 2-5

A casting was produced by following the procedure EXAMPLE 2-5 except ause of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting.

The results obtained by evaluating characteristics of the casting areshown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Casting Condition                 Cast Product Characteristics                Molten Metal Composition          Tensile          Hard-                      (wt %)                     Pressurizing                                                                         Strength                                                                           Endurance                                                                           Elongation                                                                          ness                       JIS       Si                                                                              Fe                                                                              Cu                                                                              Mn Mg Ti                                                                              Al Pattern                                                                              (MPa)                                                                              (MPa) (%)   (HB)                                                                              Product                __________________________________________________________________________    Example                                                                         .sup. 1                                                                          AC4CH                                                                              7.0                                                                             0.1                                                                             --                                                                              -- 0.3                                                                              0.1                                                                             bal.                                                                              80%   300  230   12    80  Alminum wheel          2-1  AD4CH                                                                              7.0                                                                             0.1                                                                             --                                                                              -- 0.3                                                                              0.1                                                                             bal.                                                                              30%   300  250   15    80  Alminum wheel          2-2  AC7A --                                                                              0.1                                                                             --                                                                              0.1                                                                              5.0                                                                              --                                                                              bal.                                                                              20%   300  200   20    60  Corrosion                                                                     resistant part         2-3  AC4B 8.0                                                                             0.3                                                                             3.0                                                                             0.2                                                                              0.2                                                                              0.1                                                                             bal.                                                                              30%   300  250    4    90  Engine block           2-4  AC1B --                                                                              0.1                                                                             4.0                                                                             -- 0.2                                                                              0.1                                                                             bal.                                                                              30%   420  380   10    100 Automobile                                                                    thick part             2-5  2014 0.7                                                                             0.2                                                                             4.5                                                                             1.0                                                                              0.5                                                                              0.1                                                                             bal.                                                                              20%   350  300   15    90  Large size                                                                    tough part             Compar-                                                                       ative Ex.                                                                       .sup. 1                                                                          AC4CH                                                                              7.0                                                                             0.1                                                                             --                                                                              -- 0.3                                                                              0.1                                                                             bal.                                                                             100%   280  220   10    80  Alminum wheel          2-1  AD4CH                                                                              7.0                                                                             0.1                                                                             --                                                                              -- 0.3                                                                              0.1                                                                             bal.                                                                             100%   280  250   10    80  Alminum wheel          2-2  AC7A --                                                                              0.1                                                                             --                                                                              0.1                                                                              5.0                                                                              --                                                                              bal.                                                                             100%   270  200   10    50  Corrosion                                                                     resistant part         2-3  AC4B 8.0                                                                             0.3                                                                             3.0                                                                             0.2                                                                              0.2                                                                              0.1                                                                             bal.                                                                             100%   260  250    2    90  Engine block           2-4  AC1B --                                                                              0.1                                                                             4.0                                                                             -- 0.2                                                                              0.1                                                                             bal.                                                                             100%   380  350    7    100 Automobile                                                                    thick part             2-5  2014 0.7                                                                             0.2                                                                             4.5                                                                             1.0                                                                              0.5                                                                              0.1                                                                             bal.                                                                             100%   300  250   10    90  Large size                                                                    tough                  __________________________________________________________________________                                                           part               

As shown in Table 1, each product of examples according to thisinvention shows superior characteristics as compared with each ofcomparative examples, particularly the inventive samples exhibitsuperior tensile strength and elongation.

EXAMPLE 3-1

Using the device of FIG. 13, an aluminum wheel was cast by pressurizingup to 6 kgf/cm² using aluminum alloy molten metal of Al--Si--Mgcomposition with P1 as 27% of P5 according to the pressure controlpattern shown in FIG. 7. The holding furnace side was previously applieda pressure of 0.15 kgf/cm² so that the molten metal free surface waspositioned near the sprue of the casting mold in the molten metalfeeding pipe before starting the casting. The holding furnace sidepressure was lowered to a pressure 0.15 kgf/cm² higher than the castingmold side pressure from T5 to T6, thereby the molten metal was returnedso that the molten metal free surface was positioned near the sprue ofthe casting mold in the molten metal feeding pipe. After T6, thepressures in both containers were reduced discharging the gas in thecontainers to the atmosphere. The pressure in the holding furnace sidecontainer was reduced to keep a pressure of 0.15 kgf/cm² so that themolten metal free surface is held statically.

Table 2 shows the results of evaluation of the casting.

EXAMPLE 3-2

Using the device shown in FIG. 13, a decorative part which was requiredto have corrosion resistance was cast from aluminum alloy molten metalof Al--Mg composition with P1 set to 15% of P5 according to the pressurecontrol pattern shown in FIG. 7.

Table 2 shows the results of evaluation of the casting.

EXAMPLE 3-3

Using the device shown in FIG. 13, an automobile engine block was castfrom aluminum alloy molten metal of Al--Si--Cu composition with P1 setto 15% of P4 according to the pressure control pattern shown in FIG. 8.

Table 2 shows the results of evaluation of the casting.

EXAMPLE 3-4

Using the device shown in FIG. 13, an automobile thick-wall part wascast from aluminum alloy molten metal of Al--Cu composition with P1 setto 17% of P4 according to the pressure control pattern shown in FIG. 9.

Table 2 shows the results of evaluation of the casting.

EXAMPLE 3-5

Using the device shown in FIG. 13, a large tough part was cast fromaluminum alloy molten metal of Al--Cu--Mg composition with P1 set to 15%of P6 according to the pressure control pattern shown in FIG. 10. Table2 shows the results of evaluation of the casting.

COMPARATIVE EXAMPLE 3-1

A casting was produced by following the procedure Example 3-1 except ause of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting. Table 2 shows the results of evaluation of thecasting.

COMPARATIVE EXAMPLE 3-2

A casting was produced by following the procedure of Example 3-2 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting. Table 2 shows the results of evaluation of thecasting.

COMPARATIVE EXAMPLE 3-3

A casting was produced by following the procedure of Example 3-3 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting, Table 2 shows the results of evaluation of thecasting.

COMPARATIVE EXAMPLE 3-4

A casting was produced by following the procedure of Example 3-4 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting. Table 2 shows the results of evaluation of thecasting.

COMPARATIVE EXAMPLE 3-5

A casting was produced by following the procedure of Example 3-5 excepta use of pressure control pattern of FIG. 18, which shows a conventionalcounter pressure casting. Table 2 shows the results of evaluation of thecasting.

                                      TABLE 2                                     __________________________________________________________________________    Casting Condition                 Cast Product Characteristics                Molten Metal Composition          Tensile          Hard-                      (wt %)                     Pressurizing                                                                         Strength                                                                           Endurance                                                                           Elongation                                                                          ness                       JIS       Si                                                                              Fe                                                                              Cu                                                                              Mn Mg Ti                                                                              Al Pattern                                                                              (MPa)                                                                              (MPa) (%)   (HB)                                                                              Product                __________________________________________________________________________    Example                                                                       3-1  AD4CH                                                                              7.0                                                                             0.1                                                                             --                                                                              -- 0.3                                                                              0.1                                                                             bal.                                                                              27%   300  250   15    80  Alminum wheel          3-2  AC7A --                                                                              0.1                                                                             --                                                                              0.1                                                                              5.0                                                                              --                                                                              bal.                                                                              15%   300  200   20    60  Corrosion                                                                     resistant part         3-3  AC4B 8.0                                                                             0.3                                                                             3.0                                                                             0.2                                                                              0.2                                                                              0.1                                                                             bal.                                                                              15%   300  250    4    90  Engine block           3-4  AC1B --                                                                              0.1                                                                             4.0                                                                             -- 0.2                                                                              0.1                                                                             bal.                                                                              17%   420  380   10    100 Automobile                                                                    thick part             3-5  2014 0.7                                                                             0.2                                                                             4.5                                                                             1.0                                                                              0.5                                                                              0.1                                                                             bal.                                                                              15%   350  300   15    90  Large size                                                                    tough part             Compar-                                                                       ative Ex.                                                                     3-1  AD4CH                                                                              7.0                                                                             0.1                                                                             --                                                                              -- 0.3                                                                              0.1                                                                             bal.                                                                             100%   280  250   10    80  Alminum wheel          3-2  AC7A --                                                                              0.1                                                                             --                                                                              0.1                                                                              5.0                                                                              --                                                                              bal.                                                                             100%   270  200   10    50  Corrosion                                                                     resistant part         3-3  AC4B 8.0                                                                             0.3                                                                             3.0                                                                             0.2                                                                              0.2                                                                              0.1                                                                             bal.                                                                             100%   260  250    2    90  Engine block           3-4  AC1B --                                                                              0.1                                                                             4.0                                                                             -- 0.2                                                                              0.1                                                                             bal.                                                                             100%   380  350    7    100 Automobile                                                                    thick part             3.5  2014 0.7                                                                             0.2                                                                             4.5                                                                             1.0                                                                              0.5                                                                              0.1                                                                             bal.                                                                             100%   300  250   10    90  Large size                                                                    tough                  __________________________________________________________________________                                                           part               

As shown in Table 2, each product of examples according to the presentinvention shows superior characteristics as compared with each ofcomparative examples, particularly the inventive samples, exhibitsuperior tensile strength and elongation.

EXAMPLE 4

By using the counter pressure casting device shown in FIG. 13, thealuminum wheel shown in FIG. 14 was cast by being pressurized up to 6kgf/cm² according to the pressure control pattern shown in FIG. 11.Thermocouples 6 were disposed at each position of the casting mold 4corresponding to the positions S1, S3, S4, D1 and D3 of the productshown in FIG. 14, so that the measured temperatures were inputted in thepressure control device 7. The pressures in the casting mold sidecontainer 1 and holding furnace side container 2 were set based on themeasured temperature information.

More specifically, as shown in FIG. 15, the casting mold side container1 was kept at the atmospheric pressure, while the holding furnace sidecontainer 2 alone was pressurized to P1 (1.5 kgf/cm²) by the pressuremeans 9, thereby the molten metal was fed into the casting mold 4, thenthe pressure in the holding furnace side container 2 was kept constantfrom T2 to T3 after charging of the molten metal into the casting moldwas confirmed by the thermocouples 6 mentioned above. Then, till T4 fromT3 when the temperature measured by the thermocouple 6 disposed at theposition S3 started to decrease, pressures in both containers 1 and 2were started to be increased simultaneously holding the differentialpressure of 1.5 kgf/cm² between the both containers.

Then, at T4, pressure increase in both containers was stoppedsimultaneously to maintain the differential pressure of 1.5 kgf/cm²between both container 1 and 2 for a certain time. Then, after T5, theholding furnace side pressure was lowered to a pressure 0.15 kgf/cm²higher than the casting mold side pressure so that the molten metal wasreturned to allow the molten metal free surface to position near thesprue of the casting mold in the molten metal feeding pipe. After T6,the pressures in both containers were reduced discharging tile gas inthe containers to the atmosphere. The pressure in the furnace sidecontainer was reduced to keep a pressure of 0.15 kgf/cm² so that themolten metal free surface is held statically.

FIG. 16 is a photograph showing a cross sectional microstructure of thinwall portion of the product.

COMPARATIVE EXAMPLE 4

A casting was produced by following the procedure of EXAMPLE 4 exceptthat the pressure control pattern shown in FIG. 3 was used. FIG. 17 is aphotograph showing a cross sectional microstructure of the thin wallportion of the product.

As shown in FIG. 17, concentration of casting defects is recognized inthe product of the comparative example, while such a concentration ofcasting defects is not recognized in the product of the example as shownin FIG. 16.

According to the counter pressure casting of EXAMPLE 4 of thisinvention, after charging the molten metal into the casting mold, thecasting mold side container is still set to be retained at a lowpressure compared with the holding furnace side pressure before thepressures in the both containers start to be increased simultaneously.This contributes to the prevention of concentration of casting defectsto a thin-wall portion whose solidification completes at the early stageof casting.

Further, after solidification of the thin-wall portion, both containersare pressurized forming the differential pressure between bothcontainers, so that a good casting with less casting defects is obtainedat the thick wall portion.

As described above, according to the counter pressure casting andcounter pressure casting device of this invention, at the start, of themolten metal charging into the casting mold, the pressures in thecasting mold side container and the holding furnace side container arecontrolled to low pressures compared with the maximum pressures by thecontrol means. This results in the following effects:

(1) Shortening of the casting cycle time and improvement of productivityof the counter pressure casting for its industrial application.

(2) A casting with less casting defects or less non-metallic inclusioncan be obtained even when producing a thick- or thin-wall casting with acomplicated shape or using a material that is difficult to cast.

Further, since the differential pressure increasing speed between thefurnace side container and the casting mold side container is made to bevariable in the molten metal charging process, the following effects canbe obtained:

(3) A casting with less casting defects can be obtained even whenproducing a thick- or thin-wall casting with a complicated shape orusing a material that is difficult to cast.

In addition, a pressure higher than atmospheric pressure is set to beapplicable to the holding furnace side container to position the moltenmetal surface near the sprue of the casting mold in the molten metalfeeding pipe by the pressure control means. This affects as follows:

(4) The casting cycle can be greatly shortened because the distancebetween the molten metal surface and the casting mold is shortened andthe productivity can be improved.

(5) The quality of casting is improved and dispersion of quality iseliminated because the casting condition is always uniform.

(6) The molten metal stays near the sprue of the casting mold in themolten metal feeding pipe even after the release of pressures in bothcontainers, so that stirring of the molten metal in the furnace due tothe reverse flow of the molten metal is avoided resulting in preventionof inclusion of gas and oxide into the molten metal.

(7) The feeding speed into the casting mold can be made slow, so thatthe casting defects due to the occurrence of turbulence at feeding canbe prevented.

(8) The molten metal separation becomes good when the molten metal isreturned into the furnace as the neighborhood of the sprue of thecasting mold is always heated by the molten metal.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A method for counter pressure casting moltenmetal in an apparatus,said apparatus comprising (1) a casting mold sidepressure container having therein a casting mold and (2) a holdingfurnace side pressure container having therein a molten metal-containingfurnace, said casting mold and said molten metal-containing furnacebeing in communication via a molten metal feeding pipe, said methodcomprising the steps of, setting the pressure in the holding furnaceside pressure container at a level higher than the pressure in thecasting mold side pressure container, to thereby charge molten metalfrom the molten metal-containing furnace into the casting mold via themolten metal feeding pipe, wherein the pressures in both containers arelower than the maximum pressures of the respective containers,increasing the pressure in the holding furnace side pressure containerand the pressure in the casting mold side pressure container,maintaining a pressure differential at a certain level between thepressure in the holding furnace side pressure container and the pressurein the casting mold side pressure container, dissolving the pressuredifferential between the pressure in the holding furnace side pressurecontainer and the pressure in the casting mold side pressure container,and reducing the pressure in the holding furnace side pressure containerand the pressure in the casting mold side pressure container.
 2. Themethod for counter pressure casting molten metal of claim 1,wherein thecasting mold comprises a sprue which connects the casting mold and themolten metal feeding pipe, and wherein the pressure in the holdingfurnace side pressure container is maintained a level slightly higherthan atmospheric pressure, so that the surface of the molten metal inthe molten metal feeding pipe is no lower than slightly below the sprue.3. The method for counter pressure casting molten metal of claim1,wherein said step of setting the pressure comprises increasing thepressure in the holding furnace side pressure container.
 4. The methodfor counter pressure casting molten metal of claim 1,wherein said stepof setting the pressure comprises increasing the pressure in the holdingfurnace side pressure container while maintaining the pressure in thecasting mold side pressure container at atmospheric pressure.
 5. Themethod for counter pressure casting molten metal of claim 1,wherein saidstep of setting the pressure comprises lowering the pressure in thecasting mold side pressure container.
 6. The method for counter pressurecasting molten metal of claim 1,wherein said step of setting thepressure consists of, (1) generating and initially increasing a pressuredifferential between the pressure in the holding furnace side pressurecontainer and the pressure in the casting mold side pressure container,and (2) further increasing the pressure differential at a greater ratethan the initial increasing.
 7. The method for counter pressure castingmolten metal of claim 1,wherein the pressure differential between thepressure in the holding furnace side pressure container and the pressurein the casting mold side pressure container in said step of setting thepressure, is varied to form a non-linear curve when described as apressure-time curve.
 8. The method for counter pressure casting moltenmetal of claim 1,wherein at the beginning of the step of setting thepressure, the pressure in the holding furnace side pressure containerand the pressure in the casting mold side pressure container are set at0 to 50% of the maximum pressure in the respective containers.
 9. Themethod for counter pressure casting molten metal of claim 1, furthercomprising the step ofholding the casting mold side pressure containerat atmospheric pressure or a low pressure of 3 kfg/cm² or less for aperiod of time between said step of setting the pressure and said stepof increasing the pressure.
 10. The method for counter pressure castingmolten metal of claim 1, further comprising, after said step of settingpressure, the step ofholding said holding furnace side pressurecontainer and said casting mold side pressure container at a certainpressure for a certain time, to complete solidification of the casting.11. The method for counter pressure casting molten metal of claim1,wherein during said pressure increasing step, said pressuredifferential between the pressure in the holding furnace side pressurecontainer and the pressure in the casting mold side pressure containeris held constant.
 12. The method for counter pressure casting moltenmetal of claim 1,wherein during said pressure increasing step, saidpressure differential between the pressure in the holding furnace sidepressure container and the pressure in the casting mold side pressurecontainer is increased.
 13. The method for counter pressure castingmolten metal of claim 1,wherein during said pressure differentialmaintaining step, said pressure differential between the pressure in theholding furnace side pressure container and the pressure in the castingmold side pressure container is varied.
 14. The method for counterpressure casting molten metal of claim 1,wherein during said pressuredifferential maintaining step, said pressure differential between thepressure in the holding furnace side pressure container and the pressurein the casting mold side pressure container is varied depending on thedesired condition of the casting.
 15. The method for counter pressurecasting molten metal of claim 1,wherein during said pressuredifferential maintaining step, said pressure differential between thepressure in the holding furnace side pressure container and the pressurein the casting mold side pressure container is from 0.5 to 5 kgf/cm².